The objective of these studies is to investigate brain mechanisms of selective attention in man by recording event-related brain potentials from the scalp during the execution of sensory-tumor performance tasks. By examining changes in stimulus-evoked brain potentials as subjects shift their attention between different categories of stimuli (visual, auditory or somatosensory), we hop to determine the timing and localization of the selective routing of sensory information in the brain. These data will be used to evaluate the validit of competing psychological models of attention. We have identified the auditory-evoked N1 wave (onset latency at 50-80 msec) as a correlate of an early stage of selection between different channels of input while the P3 wave (onset at 150-200 msec) reflects the selection of a significant target stimulus at a later stage. Current experiments demonstrate that common, modality non-specific mechanisms are employed when attention is shifted among different locations in extrapersonal space; visual and auditory stimuli are processed concurrently when attention is directed to either one. In other experiments, specific brain waves were found to be associated with attended language messages, both in spoken and written form. We anticipate that these waves will provide objective measures of a person's attentive capabilities and will become useful in the characterization and diagnosis of the disorders of attention which accompany different psychopathological syndromes.